We present the measurement of two event samples induced by atmospheric v(mu) of average energy (E) over bar(v) similar to 4 GeV. In the first sample, a neutrino interacts inside the MACRO detector producing an upward-going muon leaving the apparatus. The ratio of the number of observed to expected events is 0.57 +/- 0.05(stat) +/- 0.06(syst) +/- 0.14(theor) with an angular distribution similar to that expected from the Bartol atmospheric neutrino flux. The second is a mixed sample of internally produced downward-going muons and externally produced upward-going muons stopping inside the detector. These two subsamples are selected by topological criteria; the lack of timing information makes it impossible to distinguish stopping from downgoing muons. The ratio of the number of observed to expected events is 0.71 +/- 0.05(stat) +/- 0.07(syst) +/- 0.18(theor). The observed deficits in each subsample is in agreement with neutrino oscillations, although the significance is reduced by the large theoretical errors. However, the ratio of the two samples causes a large cancellation of theoretical and of some systematic errors. With the ratio, we rule out the no-oscillation hypothesis at 95% c.l. Furthermore, the ratio tests the pathlength dependence of possible oscillations. The data of both samples and their ratio favor maximal mixing and Delta m(2) similar to 10(-3)-10(-2) eV(2). These parameters are in agreement with our results from upward throughgoing muons, induced by v(mu) of much higher energies. (C) 2000 Elsevier Science B.V. All rights reserved.

We present the measurement of two event samples induced by atmospheric v(mu) of average energy (E) over bar(v) similar to 4 GeV. In the first sample, a neutrino interacts inside the MACRO detector producing an upward-going muon leaving the apparatus. The ratio of the number of observed to expected events is 0.57 +/- 0.05(stat) +/- 0.06(syst) +/- 0.14(theor) with an angular distribution similar to that expected from the Bartol atmospheric neutrino flux. The second is a mixed sample of internally produced downward-going muons and externally produced upward-going muons stopping inside the detector. These two subsamples are selected by topological criteria; the lack of timing information makes it impossible to distinguish stopping from downgoing muons. The ratio of the number of observed to expected events is 0.71 +/- 0.05(stat) +/- 0.07(syst) +/- 0.18(theor). The observed deficits in each subsample is in agreement with neutrino oscillations, although the significance is reduced by the large theoretical errors. However, the ratio of the two samples causes a large cancellation of theoretical and of some systematic errors. With the ratio, we rule out the no-oscillation hypothesis at 95% c.l. Furthermore, the ratio tests the pathlength dependence of possible oscillations. The data of both samples and their ratio favor maximal mixing and Delta m(2) similar to 10(-3)-10(-2) eV(2). These parameters are in agreement with our results from upward throughgoing muons, induced by v(mu) of much higher energies. (C) 2000 Elsevier Science B.V. All rights reserved.